This invention pertains to systems and methods for cardiac rhythm management and, in particular, for processing sensed depolarization waveforms produced by the electrical activity of the heart.
The human heart normally maintains its own well-ordered intrinsic rhythm through the generation of stimuli by pacemaker tissue that results in a wave of depolarization that spreads through specialized conducting tissue and then into and throughout the myocardium. The well-ordered propagation of electrical depolarizations through the heart causes coordinated contractions of the myocardium that results in the efficient pumping of blood. In a normally functioning heart, stimuli are generated under the influence of various physiological regulatory mechanisms to cause the heart to beat at a rate that maintains cardiac output at a level sufficient to meet the metabolic needs of the body. Abnormalities of excitable cardiac tissue, however, can lead to abnormalities of heart rhythm by affecting either impulse generation or propagation. Since such arrhythmias can be hemodynamically compromising and predispose to thromboembolic events, therapeutic intervention is usually warranted.
One therapeutic modality for treating certain types of arrhythmias is an implantable cardiac rhythm management device that delivers therapy to the heart in the form of electrical stimuli. Such implantable devices include cardiac pacemakers that deliver timed sequences of low energy pacing pulses to the heart via one or more electrodes disposed in or about the heart in response to sensed cardiac events and lapsed time intervals. Pacemakers are often used to treat patients with bradycardia and atrioventricular conduction defects. Cardiac rhythm management systems also include cardioverter/defibrillators (ICD""s) that are capable of delivering higher energy electrical stimuli to the heart and are often used to treat patients prone to fibrillation and other tachyarrhythmias. A defibrillator delivers a high energy electrical stimulus or shock to the heart to depolarize all of the myocardium and render it refractory in order to terminate the arrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. ICD""s are often combined with a pacemaker capable of pacing the heart in such a manner that the heart rate is slowed, a pacing mode referred to as anti-tachyarrhythmia pacing (ATP). ATP therapy includes a number of different protocols for delivering pulses to the heart which tend to disrupt reentrant circuits responsible for the arrhythmia. In addition to ICD""s and pacemakers, cardiac rhythm management devices also include drug delivery devices, and any other implantable or external devices for diagnosing, monitoring, or treating cardiac arrhythmias.
Since cardiac rhythm management devices are often configured to be capable of delivering a number of different electrotherapies to the heart, it is useful for the device to be programmed to recognize particular arrhythmias. That is, if an arrhythmia can be classified as a type known to be amenable to a certain therapeutic mode capable of being delivered by the device, the arrhythmia can be treated more effectively. One way of characterizing an arrhythmia is by the abnormal depolarization complex that results as the wave of excitation spreads through the myocardium during a single heartbeat. Furthermore, some depolarization complexes may represent arrhythmogenic conditions that predispose to the development of an arrhythmia. If such a condition can be recognized, preventive therapy can be delivered before the arrhythmia occurs. It is toward the objective of classifying such depolarization complexes that the present invention is primarily directed.
In accordance with the invention, a cardiac depolarization complex is sensed by a plurality of separate electrodes that sense depolarization waveforms occurring at different areas of the heart. The sensed waveforms are then compared to the corresponding depolarization waveforms of a template depolarization complex, where the template depolarization complex may be representative of an arrhythmogenic condition. If the sensed and template waveforms are judged to be similar enough, the sensed depolarization complex can be classified as being equivalent to the template depolarization complex. In order to assess the similarity of the template and depolarization complexes, the waveforms corresponding to each of the plurality of sensing electrodes can be treated as different components of a multi-dimensional vector. A multi-dimensional correlation operation is performed between the vector for the sensed depolarization complex and the vector for the template depolarization complex. The similarity between the template and sensed complexes can then be assessed by comparing the multi-dimensional correlation value to a specified value such that if the value is exceeded, the two complexes are regarded as equivalent. One or more parameter feature sets may be incorporated into to the multiple-dimension correlation to further enhance its ability to classify depolarization complexes, where a parameter feature set is a set of deviations from a mean of measurable parameters related to the complex. Each parameter in the feature set may be treated as a sample of a new sensed waveform that is given a new orthogonal direction in the vector waveform.